A whopping 10 million people play John Romero's Facebook game Ravenwood Fair every month.
It's so successful, a sequel-sized expansion, called Ravenwood Mine, is in the works, Venture Beat reports.
Ravenwood Fair, published by LoLapps and created by Romero, launched six months ago. In December it enjoyed 4.4 million average daily users.
Ravenwood Mine, also designed by Romero, will expand the game,
"We are using a traditional strategy in games," LOLapps chief executive Arjun Sethi said.
"Instead of focusing on how much money we can make from a user, we are focusing on how long we can hang on to that user. Here's a new world that interacts with the one the user has been spending a lot of time in."
In October last year Doom, Quake and Wolfenstein co-creator John Romero emerged from the wilderness as the creative force behind Ravenwood Fair.
Romero acted as a consultant on the game, the first he has worked on that has been released in years.
Romero co-founded id Software with John Carmack, Adrian Carmack and Tom Hall, and designed some of the most influential games of all time.
Romero left to start Ion Storm and created the controversial Daikatana. In 1997 Romero appeared on an advert for the game that said: "John Romero's About To Make You His Bitch....Suck it down." That didn't go down well, and some 10 years later Romero apologised to fans for it.
He then went on to form mobile game developer Monkeystone Games. After leaving that company, he joined Midway Games in 2003. He left two years later.
Now Romero is boss of his own studio: Loot Drop, which has recently signed to develop a title for social gaming company RockYou.